Copper coating compositions



United States Patent 3,141,780 COPPER CQATING COOSITIQNS John G. Simon, St. Paul, and John G. Songas, Circle Pine,

Minn, assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn, a corporation of Delaware No Drawing. Filed Mar. 30, 1962, Ser. No. 183,755 6 Claims. (Cl. 106-1) This invention relates to the copper coating of ferrous metal surfaces by depositing copper on the ferrous surface from an aqueous coating bath or solution containing copper ions. More particularly, this invention relates to new chemical admixtures and procedures for their use which enhance the usefulness of such coating baths or solutions.

Flash coating of copper from an aqueous solution onto a ferrous metal surface has found Wide use in the coating of billets and the like from which wires or other objects are drawn, the copper coating serving as a lubricant for the drawing operation. More recently these fiash coatings have been used as anchor or primer layers to facilitate mechanical plating. In the preparation of such coatings, the bath is composed essentially of an aqueous solution containing a copper compound soluble in the solution, and a free acid such as sulfuric, nitric, hydrochloric, and the like. The ferrous metal surfaced object to be copper coated is immersed in the solution and the copper ions in the solution migrate to the ferrous metal surfaces, forming continuous coating deposits thereon of elemental copper.

Because of low adherence of such copper coatings to the metal substrate when using simple coating baths of the type noted, the addition of strong, ferrous metal corrosion inhibitors to the baths has been proposed to increase adherence, as for example those disclosed in Patent No. 2,217,921. In Patent No. 2,472,393 it is proposed to utilize, not a ferrous metal corrosion inhibitor, but an intermediate molecular weight (about 4,000), polyethylene glycol as a means to obtain more adherent coatings. Further, the addition of halide ions, particularly those of chlorine and bromine, are known to enhance the rate at which copper coating proceeds and to provide the copper coating deposited with a bright finish; thus, it has been customary to include ionizable halogen containing compounds in the bath.

With the advent of mechanical plating of metal powders such as zinc, cadmium, tin, and various alloys and mixtures thereof with one another and with lead, etc. onto ferrous metal surfaces, it has been found that these mechanical platings adhere only weakly to bare ferrous metal surfaces. They do, however, anchor much more strongly to such surfaces which have first been subjected to a flash coating of copper by immersion of the ferrous metal surfaced article to be mechanically plated in a copper coating bath or solution of the nature discussed hereinbefore than to bare ferrous metal surfaces. However, a severe drawback in the utilization of such copper coatings for ferrous metal substrates to be subsequently thickly mechanically overplated to galvanizing thicknesses was discovered when it was found that no more than about half of the total surface area normally coated by such baths adhered sufficiently strongly to the ferrous metal surfaces to serve as an anchoring coating for subsequently applied heavy mechanical platings and that even with thin coatings as the bath approached being spent, the coatings become less adherent than desired for mechanical plating. Thus, these baths possessed coating lives too short for such use.

This invention provides a chemical admixture and procedures for use thereof for a copper plating bath or solution which greatly extends the life of the bath in the 3,141,780 Patented July 21, 1964 "ice production of copper anchor coatings useful for subsequently applied mechanical platings, continuing to provide tightly adherent copper anchor coatings for the subsequent application of thick mechanical platings as the bath becomes spent.

By mechanical plating or mechanically deposited platings, or similar term, as used herein, is meant a plating or coating formed by mechanical forces which coalesce metal powder particles into a coherent and adherent surface plating by hammering or otherwise physically distorting the particles into a continuous plating on an article surface. Methods for mechanically plating are well known in the art and procedures for practicing the art of mechanical plating may be found, for example, in Reissue Patent No. 23,861, and Patent No. 2,689,808.

Heretofore, because of the inability to formulate copper plating baths which would, throughout the normal life of the plating bath, produce copper coatings having sufficient adherence to ferrous metal substrates to serve as anchor coatings for heavy overplating, it has not been feasible to form mechanical platings of galvanizing thicknesses, e.g., thicknesses of about .001 inch to about .003 inch, and thicker, on ferrous metal substrates.

The chemical admixtures of the present invention provide plating baths which continue to provide flash copper coatings on ferrous metal substrates with firm adherence to such substrates substantially throughout the normal life of the copper coating bath, and in many instances beyond the normal life expectancy of the bath. These compositions now make the mechanical plating of zinc, and other usefully mechanically applied platings in galvanizing thicknesses, economically feasible. In addition, these admixtures provide superior coating baths for other purposes for which such coating baths are useful by their ability to extend bath life without loss of coating adherence or anchorage to the substrate.

A copper plating bath made in accordance with this invention, in addition to containing a copper compound, a free acid, and water, further contains a nitrogenous organic ferrous metal corrosion inhibitor of the type disclosed in Patent No. 2,217,921, and a water soluble, high molecular weight polyethylene glycol having a molecular weight of no less than about 15,000. A polyethylene glycol of 20,000 molecular weight has been found eminently suitable.

The inclusion of a high molecular weight polyethylene glycol along with a nitrogenous basic organic ferrous metal corrosion inhibitor in a copper plating bath has been found to provide a plating bath which provides tightly adherent copper coatings on large total surface areas of ferrous metal substrates, which coatings are sufiiciently well anchored to anchor thereon galvanizing thicknesses of mechanical platings. Consequently, utilizing prior known copper coating baths it was possible with a 200 gallon bath to coat no more than 1000 sq. ft. of ferrous metal surfaces having copper coatings suitable for anchor coatings for mechanical platings of galvanizing thicknesses, as opposed to a normal bath life of possibly 8,000 sq. ft. of coated surface area for billet and other coatings. It is now possible, using the chemical admixtures of this invention, to provide a copper coating bath which provides more adherent copper coatings initially and which continues to provide more adherent copper coatings as the bath is depleted, copper coatings at the end of the recommended bath life (about 4,000 sq. ft. coated p-arts/ gallon of bath) having approximately the same adherence to their ferrous metal substrates as those coatings applied while the bath was still fresh and undepleted.

No such similar effects have been observed when either the nitrogenous basic organic ferrous metal corrosion inhibitors or the high molecular weight polyethylene glycol is absent from the copper coating solution.

The copper ions of the coating bath may be provided by any cuprous or cupric compound soluble in the bath, as for example copper sulfate, cupric chloride, cuprous chloride, cupric nitrate, cupric acetate, cupric benzoate, cupric oxide, cuprous oxide, cupric oxide, etc. A presently preferred copper compound, because of its ready availability, is cupric sulfate pentahydrate.

The acidity of the bath may be provided by any hydrogen containing, strong, inorganic acid such as hydrochloric, nitric, phosphoric, eta; however, sulfuric acid is preferred.

Any of the strong, nitrogenous, organic basic inhibitors disclosed in Patent No. 2,217,921 are considered applicable to the present invention. Specific examples of such compounds are dimethyl naphthoquinaldine, 'anhydroformaldehyde aniline, tetramethyl-diamino diphenylmethane, condensation products of aldehyde, e.g., formaldehyde, acetaldehyde, and o-toluidine, salts of such condensation products, etc. Presently preferred inhibitor is the formaldehyde o-toiuidine condensation product, the formation of which is accelerated in the presence of hydrochloric acid.

The foregoing are all of the ingredients, outside of the water of course, necessary in the preparation of a copper plating bath that will function in accordance with this invention to provide copper anchor coatings for ferrous metal substrates capable of being overplated with thick mechanical platings. The consistently good quality of copper coatings throughout the life of the bath makes the chemical admixtures of this invention of considerable value in any field where copper coatings on metal substrates are used. This invention enables continuously good adherence of cop-per coatings deposited from nearly spent, as well as fresh coating baths, onto ferrous metal substrates; and, thus, the present invention is also useful in the preparation of metal billets prior to drawing and the like as well as in other areas than mechanical plating where copper coatings are desired.

Particularly in those instances where the copper coating is to serve as a finish coating, and preferably in all instances, it is well to include a source of halogen ions in the plating bath as the presence of these ions therein enhances the rapidity with which copper is deposited from solution and causes the copper to form bright, very smooth appearing coatings. Halide ions can be provided by common table salt, anunonium chloride, calcium chloride, potassium chloride, or the corresponding bromides and such other compounds, including an acid such as HCl, which are soluble in the water of the bath.

In the preparation of a copper plating bath in accordance with this invention, in parts by weight, for each two parts copper (elemental weight) in the bath, there should be at least about /3 part high molecular weight polyethylene glycol and excellent results are achieved with about 1 part glycol for each two parts copper. While the amount of high molecular weight polyethylene glycol possible for use with the copper may be so high as to produce a polyethylene glycol saturated solution, as a practical matter of economics from about /3 of a part to about 1 part for each two parts copper is adequate.

When a strong ferrous metal corrosion inhibitor such as the condensation product of orthotoluidine and formaldehyde is used, it should be present in about 0.8 part to about 1.2 parts for each two parts elemental copper. Too little of the inhibitor produces coatings which do not adhere well and too much of the inhibitor produces coating baths from which the copper does not readily deposit. With a strong inhibitor of the nitrogenous basic organic type noted, the limits are rather narrow to produce good coating as can be seen; however, with weaker inhibitors these limits may be broadened considerably. When using copper sulfate pentahydrate as the copper ion source in the bath, the amount of inhibitor can be readily determined by simple addition of the inhibitor to the bath until the color of the bath changes from blue to green indicating conversion from cupric to cuprous ions. In the event too much inhibitor is added, the solution changes color to a yellowish-brown and the copper does not readily deposit therefrom.

In the event a halide ion providing salt is utilized, the amount is not critical and sufiicient can be used to saturate the solution with the salt; however, an amount as little as /2 part for each two parts copper will result in a brighter appearing copper coating.

A commercially useful plating bath composition is set forth in the formulation of the example following.

EXAMPLE I A ZOO-gallon bath is made up of water containing 6 gallons of 66 B. H 80 34 pounds of copper sulfate pentahydrate, 12 pounds of NaCl and 13 pounds of a concentrate in accordance with this invention listed hereafter.

Concentrate Formulation In making up the concentrate, which is a reddishbrown liquid, it is desirable to first add the polyethylene glycol and hydrochloric acid to the water and thereafter i add the ortho-toluidine and formaldehyde as otherwise considerable heat may be evolved as the ingredients react. Completion of the reaction in the concentrate formation is indicated by the formation of the final reddish-brown color, as well as completion of the exotherm. The concentrate is stable at ambient temperatures and may be packaged in containers inert to hydrochloric acid. Polyethylene lined containers are very satisfactory.

A ZOO-gallon bath made up as described has been found to adequately copper coat 8,000 square feet of ferrous metal surfaced parts for subsequent anchorage thereto of thick mechanical platings. Thus, 20,000 pounds of /8 steel washers (more than 8,000 sq. ft. of exposed surface area) were coated by immersion into the bath for from 30 to 60 seconds in 600 to 800 pound lots.

Upon removal from the bath the lots were mechanically plates with zinc powder to a plating thickness of 0.003 of an inch. Upon bending selected of the plating washers at a angle, then inspection of the plating which flakes off at the angle juncture reveals whether or not any of the copper coating has transferred from the ferrous metal substrate. Testing selected washers in this manner from each lot of the 20,000 pound batch revealed no transfer of copper to the zinc plating, indicating good copper anchorage, regardless of whether the parts had been plated from the first lot, or the last lot or any of the intermediate lots. Identical mechanically plated zinc washers subjected to prior known copper coating immersion baths generally displayed significant copper transfer after the first few thousand pounds and invariably after about 5,000 pounds (each 2 /2 lbs. of parts being about one sq. ft. of surface area).

EXAMPLE II Thirty thousand grams of assorted steel washers were immersed in 3,000 gram lots consecutively for 1 minute in 2,500 cc. coppering bath of the kind set forth in EX- ample I (this is equivalent to about 40 sq. ft. of coated surface per minute per gallon of bath).

The resultant copper'coated washers of each lot were then mechanically plated to a plating thickness of approximately 0.003 inch (galvanizing thickness) in a tumbling drum for 1% hours as follows:

Copper coated parts 3,000 gms.

Zinc powder 150 gms.

Promoter chemical 1 gms.

Tumbling barrel 4500 cc. capacity hexagonal drum; 54 rpm.

2 qt. 0304 (ns). 1 qt. 1214 (s). /2 qt. 110 (9100s).

1A mixture of an organic film former and a metal oxide solvent. Selected washers from each of the 10 lots were then tested for adhesion by bending at a 90 angle over a vise and observing the spalling of zinc on the external and interior surfaces of the bend area to ascertain whether or not there was significant copper transfer from the metal substrate to the zinc plating. While more transfer was evidenced in the last few lots of washers treated in the coppering bath than in the first several, even in the last lot the copper transfer was not significant and the lot was generally comparable to the first lot.

Coppering baths from which the nitrogenous basic organic inhibitor, or the high molecular weight polyethylene glycol, or both, had been omitted provided copper anchor coatings having good substrate adhesion only for the first few lots of coated washers. Adhesion deteriorated rapidly from the fourth or fifth 3,000 gram lot to the 10th lot as evidenced by complete separation of the copper coating from the metal substrate in the bend areas of washers subjected to the adhesion test previously described.

Comparison tests of the new plating bath of this invention with prior art plating baths are set forth in the following example.

Impact media EXAMPLE III Two plating baths were made up, both utilizing 2,000 cc. of water as follows:

A Cuprodine #2 ...J 3%(wt.). H2804 (Vol-)- B CuSO -5H O 2% (wt.). NaCl /3% (wt.). Additive of Ex. I (vol.). H 50 3% (vol.).

1 A solid sodium chloride and corrosion inhibitor containing opprzrc sulfate chemical marketed by American Chemical Immersed in each of these solutions were test specimens composed of /8 inch stamped metal washers, inch T- nuts, and one inch fiat stampings which test specimens were prepared first by giving a 5 minute soak in a commercial cleansing solvent in water at 180 F. No etch was given to any of these surfaces. The specimens were then thoroughly rinsed in cold water.

Thereafter, the specimens were immersed in each of solutions A and B for 60 seconds, rinsed, and dried.

Next, the specimens from each bath were tested for adherence of the copper coating to the ferrous metal substrate by applying the pressure sensitive adhesive of a cellophane tape to the copper surface and then mechanically stripping the tape from the surface. With respect to the parts coated in solution A, copper transfer to the adhesive surface of the tape occurred.

Applying the test to the samples immersed in solution B, no transfer of copper could be noted on the test section of the tapes even under microscopic examination.

Further, bending of each of the specimens coated in solution B about a inch radius through 90 did not produce spalling of the copper coating or transfer to the test section of the tape.

An equivalent of 100 lbs. of metal parts per gallon of solution was processed through solution B and the test results, as noted above, of the last part of the batch were found to be comparable to those of the first part of the batch.

To make up a gallon of plating bath, the following limits are recommended:

With respect to the sulphuric acid, 3% has been found to produce an optimum formation rate of the copper coating combined with the best adhesion of the coating to the substrate. Lesser quantities provide slower and poorer formations whereas larger amounts of the acid increase the rate of formation and appear to impair the adhesion properties.

From the foregoing, the invention will be apparent to those skilled in the art. What is claimed as new is as follows:

1. A copper coating applying bath, each gallon of which is capable of applying a strongly anchored copper coating to about 40 square feet of ferrous metal surface, said bath consisting essentially of an aqueous solution of about 1 to 5% of a strong inorganic acid containing an ionizable copper salt capable of plating copper out of solution onto a ferrous metal, and containing for each two parts by weight elemental copper, at least about /3 part by weight of a water-soluble inert waxy ethylene polymer having a molecular weight of at least about 15,000, and from about .8 to 1.2 parts of a strong nitrogenous organic basic ferrous metal corrosion inhibitor.

2. A copper coating applying bath, each gallon of which is capable of applying a strongly anchored copper coating to about 40 square feet of ferrous metal surface, said bath consisting essentially of an aqueous solution of about 1 to 5% of a strong inorganic acid containing an ionizable copper salt capable of plating copper out of solution onto a ferrous metal, and containing for each two parts by weight elemental copper, from /3 to 1 part water-soluble polyethylene glycol having a molecular weight of about 20,000, and from about .8 to 1.2 parts of a strong nitrogenous organic basic ferrous metal corrosion inhibitor.

3. A copper coating applying bath, each gallon of which is capable of applying a strongly anchored copper coating to about 40 square feet of ferrous metal surface, said bath consisting essentially of an aqueous solution of about 1 to 5% of a strong inorganic acid containing an ionizable copper salt capable of plating copper out of solution onto a ferrous metal, and containing for each two parts by weight elemental copper, from /3 to 1 part water-soluble polyethylene glycol having a molecular weight of about 20,000, and from about .8 to 1.2 parts condensation product of formaldehyde and o-toluidine.

4. An additive for extending the life of a copper coating bath and enhancing coating anchorage consisting essentially of an aqueous liquid concentrate having dissolved therein a water-soluble polyethylene glycol having a molecular weight of at least about 15,000 and a strong nitrogenous organic basic corrosion inhibitor.

5. An additive for extending the life of a copper coating bath and enhancing the adherence of the copper coating to ferrous metal substrates consisting essentially of, for each two parts copper in the bath, a combination of about /3 to 1 part water-soluble polyethylene glycol having a molecular weight of about 20,000, and about 0.8 to 1.2 parts of a strong nitrogenous, organic, basic ferrous metal corrosion inhibitor.

7 r a 6. A copper plating solution consisting essentially of of about 20,000, and from about .8 to 1.2 parts conan aqueous solution of a strong inorganic acid containdensation product of formaldehyde and o-toluidine.

ing an ionizable copper salt capable of releasing elemental copper from solution for deposition on ferrous metal surface, and containing for each two parts by 5 UNITED STATES PATENTS Weight of elemental copper, from /3 to one part water- 2,217,921 Saukaitis Oct. 15, 1940 soluble polyethylene glycol having a molecular Weight 2,472 393 Avallone et a1 June 7, 1942 References Cited in the file of this patent 

3. A COPPER COATING APPLYING BATH, EACH GALLON OF WHICH IS CAPABLE OF APPLYING A STRONGLY ANCHORED COPPER COATING TO ABOUT 40 SQUARE FEET OF FERROUS METAL SURFACE, SAID BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF ABOUT 1 TO 5% OF A STRONG INORGANIC ACID CONTAINING AN IONIZABLE COPPER SALT CAPABLE OF PLATING COPPER OUT OF SOLUTION ONTO A FERROUS METAL, AND CONTAINING FOR EACH TWO PARTS BY WEIGHT ELEMENTAL COPPER, FROM 1/3 TO 1 PART WATER-SOLUBLE POLYETHYLENE GLYCOL HAVING A MOLECULAR WEIGHT OF ABOUT 20,000, AND FROM ABOUT .8 TO 1.2 PARTS CONDENSATION PRODUCT OF FORMALDEHYDE AND O-TOLUIDINE. 